A major challenge to the prevention or treatment of breast cancer metastasis is to understand how tumors respond to environmental signals that regulate their invasive/migratory capacity and then develop selective therapeutic strategies to abrogate these processes. We have identified a novel pathway (the cell surface receptor tyrosine kinase discoidin domain receptor 2 (DDR2)) turned on in breast cancer cells and activated by tumor environmental signals (fibrillar collagen) that is critical for breast cancer metastasis. DDR2 is present on 70% of human invasive breast tumor cells including all clinical subtypes and also invasive DCIS. Importantly DDR2 is not expressed by normal breast epithelia. We have now identified a group of potent and selective novel small molecules that interact with the extracellular domain (ECD) of DDR2 to inhibit DDR2 binding to, and activation by collagen I. When invasive human breast cancer cell lines that express DDR2 are treated with these molecules the activation of DDR2, cell proliferation, and tumor cell invasion are inhibited. We hypothesize that the RTK, DDR2, is a new target for the treatment of breast cancer metastasis and that novel inhibitors targeting the ECD of DDR2 to prevent its activation will be effective in the prevention and, or treatment of breast cancer metastasis. To test these two hypothesis we shall first determine which DDR2 expressing cells within the tumor (epithelial and, or stroma) are critical for DDR2's action in the regulation of breast cancer metastasis and how. Then determine how our novel small molecules inhibit cellular signaling and cellular functions mediated by DDR2 in tumor cells and cancer associated fibroblasts (CAFs), and whether these molecules synergizes with TK inhibitors of DDR2 (Aim 2). Determine the molecular basis for inhibition of DDR2 activation by these novel molecules by solving the crystal structure of the ECD of DDR2 in a complex with inhibitor and ligand (Aim 3). Finally in Aim 4 we will determine if these novel inhibitors of DDR2 block breast cancer metastasis development and regression of already established metastases in vivo.